1. Field of the Invention
The invention relates to throttle opening control system and method for an internal combustion engine.
2. Description of Related Art
In internal combustion engines of motor vehicles, electronically controlled throttle valves are widely used. The electronically controlled throttle valve is driven, i.e., opened and closed, by an actuator. Thus, the degree of opening of the throttle valve (which will be called “throttle opening”) can be controlled with a high degree of flexibility or freedom, irrespective of the amount of depression of the accelerator pedal.
Generally, play or looseness exists in a driving system including, for example, a transmission and a differential gear unit. When the accelerator pedal is depressed while the vehicle is in a decelerating condition (with the accelerator pedal released), and the vehicle starts being accelerated, the direction of torque transmitted between the engine and the driving system is reversed. Upon the torque reversal, the play in the driving system is absorbed or removed. Accordingly, if the rate of increase of the engine speed at the time of the torque reversal is high, shock arising upon absorption of play in the driving system (which will be called “driving-system play-derived shock”) is transmitted to the vehicle, and is likely to be felt by the vehicle driver or operator.
In this case, if the rate of increase of the throttle opening in response to depression of the accelerator pedal is reduced, the rate of increase of the engine speed is also reduced, and acceleration shock may be suppressed. However, if the rate of increase of the throttle opening is simply reduced, the response to depression of the accelerator pedal may deteriorate (or become slow), thus causing deterioration of the drivability.
For example, JP-A-2004-124857 discloses a throttle opening control system that reduces the rate of increase of the throttle opening at the time when the direction of torque transmitted between the engine and the driving system is reversed, so as to suppress the above-mentioned driving-system play-derived shock. With this system, the rate of increase of the engine speed at the time of torque reversal can be reduced, thereby to suppress the driving-system play-derived shock while preventing deterioration of the response.
When the accelerator pedal is depressed while the vehicle is in a decelerating condition, acceleration shock may occur due to sway-back of the driving system transmitted to the vehicle, in addition to the above-mentioned driving-system play-derived shock. When the engine develops torque in response to opening of the throttle valve, and the engine torque is transmitted to the driving system, the driving system is placed in a twisted condition for a moment. Reaction force generated from the driving system when it is restored from the twisted condition causes sway-back of the driving system.
In the throttle opening control system- as disclosed in the above-identified publication, acceleration shock due to sway-back of the driving system is not taken into consideration, and there may arise a problem that the acceleration shock due to sway-back of the driving system cannot be sufficiently suppressed.